In general, unoriented polymers have a relatively large coefficient of thermal expansion, as compared to other materials such as inorganics. Furthermore, for many applications, e.g. film and fiber, most polymers are not strong enough to be used without orientation. But orientation causes an adverse effect on the resistance of a polymeric structure to permanent irreversable shrinkage. These shrinkage effects are so pronounced that they obscure the effect of reversable thermal expansion.
Irreversable shrinkage as used herein means thermally irreversable permanent dimensional change from a stressed condition as opposed to contraction which is thermally reversable.
Structures such as fibers and films having exceptional thermal dimensional stability would be exceedingly useful for use as magnetic tape bases, circuit boards, substrates for miniaturized electronics, fibers for highly accurate measurements and the like.
Polymeric materials with this unique characteristic would also be extremely useful for reinforcing fillers and for use in those situations where dissimilar materials are in contact with each other and are subject to wide variations in thermal cycling. Examples of this type of combination would be metallized polymer foils and laminates containing layers of different types of polymers and other materials.
The thermal expansion coefficient is defined at temperature T as .alpha..sub.T, where ##EQU1## Where L.sub.o is the initial sample length, and .DELTA.L is the change in sample length which occurs during the change in temperature, .DELTA.T.
Isolated instances of specific unoriented polymer structures which show a zero coefficient of expansion are described in U.S. Pat. Nos. 3,455,879 and 3,642,709. These are formed from specific polymer compositions which are totally unrelated to the ones of this invention.